Abstract : An omnidirectional hydrophone having an elastic shell in the form of an oblate ellipsoid of revolution having the ratio of its major axis to is minor axis greater than about V(2-v) where v is Poisson's ratio of the shell material, wherein the circular circumference of the shell (at different circular parallels of latitude) undergoes strains of opposite sign when the shell is subjected to a pressure change. The differential strains are advantageously measured by an optical fiber interferometer having one leg wound about the equatorial circumference of the shell and another leg spirally wound near one or both of the poles.